Processes for the manufacture of methyl alcohol from methane are described in U.S. Pat. No. 4,990,696 and U.S. Pat. No. 5,185,479, which are incorporated in their entirety by reference in this specification. In both of these references, methane was assumed to be the raw material. Thus, no provisions were made for a purge to remove inert gases that are invariably found in natural gas.
In practice, pure methane would be impractical to use for methyl alcohol production on a large scale. Its high cost and lack of availability would preclude such an application. Instead, the source of methane would generally be natural gas. Although inexpensive and abundant, natural gas presents difficulties in its use caused by the fact that it contains a number of constituents besides methane. For example, natural gas from one source was reported to contain 96 percent methane by volume, 3.2 percent nitrogen and 0.8 percent carbon dioxide. Another source contained 80.5 percent methane, 18.2 percent ethane and 1.3 percent nitrogen. Commonly, natural gas may contain anywhere from 0.1 percent to over 7 percent inerts.
Although hydrocarbons other than methane can quite easily be extracted from natural gas, inerts present a more difficult challenge. Such inerts include nitrogen and carbon dioxide. In contrast to conventional processes for producing methanol via synthesis gas, technology based on chlorination chemistry cannot handle carbon dioxide. This gas, therefore, must be treated in the same manner as other inerts, notably nitrogen.
The problem of using natural gas as a raw material for methanol production is compounded by the side reactions encountered in chlorination chemistry. In the chlorination of methane to methyl chloride, the latter compound is further chlorinated to give decreasing quantities of methylene chloride, chloroform and carbon tetrachloride. In order to maximize the yields of methyl chloride, it is necessary to use an excess of methane. This condition can be realized by recycling unreacted methane to the reactor. This procedure, however, leads to the build up of unacceptable levels of inerts in the recycle stream when natural gas is the feed.
Further complicating the recycle of methane is the need to separate hydrogen chloride gas from the recycle stream. Several methods are available for separating hydrogen chloride from methane, but each one has drawbacks. The aforementioned references disclose the use of stripper-absorber columns employing hydrochloric acid solutions to remove hydrogen chloride. The utility requirements for these units, however, are considerable. Alternative means for separating hydrogen chloride from methane include the use of gas separation membranes or adsorbents. But neither of these approaches are entirely satisfactory since they require either elevated pressures or temperatures for their operation.
It is therefore an object of the present invention to provide a process which overcomes the disadvantages of existing technology for producing methyl alcohol.
A further object is to be able to handle a wide variety of natural gas feedstocks while achieving high yields of product in an environmentally friendly process.
Still another object is to provide for a practical process by reducing investment and operating costs to a minimum.
These and other objects, features and advantages of the invention will be apparent from the following description and the accompanying drawing.